Brother is looking to purchase a new 650cc fork,

https://www.cyclesmercier.com/geometry_tt.html

that is he is geo spec sheet, He has a 47.

The stock fork
Kilo TT: 1'' Threadless Fork

Description: Fork Specifications

Steerer Tube: 1'' Threadless
Steerer Tube Length: 330 mm
Fork Rake: 44 mm
Axle to Crown Length: 345 mm
Crown Race: 26.4 mm

Front Axle Type: 9x1
Wheel Size: 700c
Front Hub Spacing: 100 mm
Brake Type: Road Caliper - Standard Reach
Brake Usage F/R: Front
Steer Tube Material: Steel


My brother has a frame builder and fork builder, willing to build his fork.

Using a rake caculator i've came to conclusion that 41.10 mm rake will be the same trail

I'll use the same dimensions on everything else correct?

What would the axle to crown be?

Not sure what the radius is for your 650 wheel, but if it's 310 mm (the 700c is about 340), the desired rake is 29 mm, not 41.

Can you explain how you got 41? Assuming r1 = 37.5 per the site, and the head angle is 73 degrees, trail as supplied is about 65 mm. Solving the trail equation for offset r2,

-r2 = T*sin 73 - R2*cos 73 = -29
r2 = 29

Just from solving the trail equation.

If you need to add 650 only in the front and keep the bike level (otherwise your head angle won't be 73 degrees), your fork needs to be longer, by the difference between the 700 and the 650 radii. Also needs a small adjustment for reduced offset. If your going 650/650, the fork needs to be shorter by the same amount to get the brake track in range of the calipers.

He wants to run 700 in the rear and 650 in the front. changing the head tube angle.

So you are ok with the head tube riding lower and the head tube being different?

Did you calc the new head angle? what do you get?

the new trail will indeed depend on the HT angle (which will change if the new fork is shorter) and on the wheel radius.

I'm not sure whether you have 37 or 44 mm rake, the spec sheet differs from your post (and by the way 345 mm is impossibly short for a 700c wheel, 365 is standard). Respectively that gives 65 or 59 mm trail.

Assuming the 650c wheel radius is 313 mm, the new fork will be approx. 335 mm a-t-c and your HT angle (and ST angle) will steep up considerably by 5 deg! And then you'd need something like 10 mm of rake to achieve 60 mm trail.

5 deg seems like a big change so perhaps someone else can confirm. But if it is correct I would NOT recommend a shorter fork. Why the change?

Barspinz yo.

Oh I get it. You want to put a 650c wheel on a 700c-sized fork but with a different rake. Still that'll drop the front considerably and should change your angles by 3 degrees. At 76 deg (!) you'd need 20 mm of rake for 60ish trail.

I think this is right. It's a heck of an angle change, will ride rather different.

Yup... a 77/76 bike will be weird.

If the OP is having a fork made, as Road Fan said make it 30mm taller (at say 395 mm a-t-c) and with a rake of 35 or 29 mm for 59 or 65 mm of trail.

Framebuilder Tom Matchak has a niche supplying forks that retain the proper frame orientation. Worth talking to.